The distance the tortoise covers in t seconds is 0.10*t meters and the distance the hare covers is 20*0.10*(t-120) meters as Sonty said. However, the condition that "the tortoise wins by 20 cm" means that, if t is the time when the tortoise crosses the finish line, then 20*0.10*(t-120)= 0.10*t- 0.2. That's the equation you need to solve.